As soon as he set eyes on his wife following surgery to mend a broken hip, Tony Hewitt-Miles could tell there was something different about her.

'Rita looked at me vaguely - as if she couldn't quite remember if she knew me or not,' says Tony, 84, from Bexhill-on-Sea, East Sussex.

'At the time I thought: “She has just had an operation.” However, when we got home she kept confusing me with her father. She also had no idea where the loos were - and we live in a two-bedroom flat.'

Joyful memories: Tony and Rita Hewitt-Miles, who have been happily married for five years. Rita however has suffered from impaired short-term memory since a surgery in which she went under general anaesthesia

Yet immediately before the operation in January 2011 Rita, now 83, had whizzed through crosswords, remembered everyone's birthdays and enjoyed reading.

Now she can't even remember where she lives. Not only that but she has lost her sense of balance and topples over without a zimmer frame.

'We went for the six-week check-up and they asked her how she was and she said: “I'm fine.” But I said: “Hang on a minute, no she's not,” ' says Tony, who is retired from his job in personnel at the Civil Aviation Authority.

'I told her doctor she had no short-term memory - it's so bad that if she goes to the loo she forgets she's been and turns to go back again.

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'He said: “Oh, that's a side-effect of the general anaesthetic, but she will be herself again within three months.”

'Well, that was two-and-half years ago and she still isn't any better.

'We went back to the GP a few months after that initial check-up, but he wasn't an awful lot of help. He asked me what I'd like him to do - well, how do I know what to suggest?

'It makes me so sad. We were both widowed and got married late in life, in 2008. We wanted to make the best of our time together.

'Before she had the operation they asked if she was ok with general anaesthesia and we said yes. She'd had surgery on her other hip ten years previously, when she was 70, and had been fine. What they didn't say was that this time, because she was older, there could be problems.'

Missed opportunities: It's been two-and-a-half years since Rita's operation, but the effects of the anaesthesia remain

In many ways, general anaesthesia has never been safer.

Back in the Forties, around 640 people in every million who had an anaesthetic died as a result. But as the drugs and anaesthetists' skills improved, the number of deaths dropped rapidly - by 90 per cent since the Seventies, according to data published in The Lancet in 2011.

So, out of the three million people who have a 'general' in the NHS each year in Britain, fewer than 20 will die as a result.

Yet many - especially older people such as Rita - find they are never quite the same again afterwards.

It's more than just feeling a little hazy for a few hours after an operation.

Significant numbers develop post-operative cognitive decline (POCD), suffering from memory loss and behaviour changes; they're also just not as sharp as before. This can last weeks or even months. Some believe it can be permanent.

But this is not the only potential effect. Recently it's been suggested that general anaesthesia may also increase the risk of dementia - possibly by causing inflammation in the brain.

Other studies have suggested it weakens the immune system.

Researchers are also looking at the possibility that in the under-threes it may lead to the death of brain cells and increase the risk of developing memory and learning difficulties.

Doctors still don't fully understand how general anaesthesia works, says Dr Robert Sanders, an honorary lecturer at the department of anaesthesia at University College London Hospital.

'General anaesthesia seems to mimic sleep, but it's more complex than that, as patients don't wake up during surgery as they would if merely asleep.'

Anaesthetic slows down the electrical activity of the brain.

'Brain cells communicate with one another with electrical impulses,' says Professor Jaideep Pandit, a consultant anaesthetist at the John Radcliffe Hospital in Oxford.

'We know that a general anaesthetic suppresses that electrical activity.

'Recently it's been suggested that
general anaesthesia may also increase the risk of dementia - possibly by
causing inflammation in the brain.'

'People are normally put under using propofol, given intravenously. It works quicker than a gas would (a gas is then used to keep them asleep).

'Some people are groggy in the minutes and hours after they wake up - we believe because the electrical activity of their brain has to speed up again,' says Professor Pandit.

Yet a recent French study suggests some people could have longer-term effects. An analysis of data from more than 9,000 men and women aged over 65 found those who'd had a general anaesthetic in the previous two years were more likely to develop dementia, including Alzheimer's disease.

One theory is that the general anaesthesia may contribute to brain inflammation and thus encourage the production of amyloid plaques, which are linked to Alzheimer's disease.

But the suggestion is hotly contested.

'The fact is that after the age of 40 we all have some cognitive decline - the findings of this study could just be normal natural decline,' says Dr Sanders.

However, there seems little question about the existence of post- operative cognitive decline.

It is most common following cardiac surgery, though as many as 40 per cent of patients over 60 get it following other forms of surgery, such as hip replacements.

The longer the surgery and the older you are, the higher the risk of developing the condition.

Cause for concern: One study found that a year after surgery, over 75% of surgery patients aged over 60 still had mild cognitive decline

While 30 to 50 per cent of people of all ages suffered from it in the first week after surgery, after three weeks that figure would drop to 10 to 15 per cent - and these would almost all be elderly people, according to a paper in Anaesthesia and Analgesia in 2011.

A study in scientific journal PLOS One last year found that 12 months after surgery, 76 per cent of patients aged over 60 who'd had orthopaedic or abdominal surgery still had mild cognitive decline, and 11 per cent had a severe case of it.

'It does become more frequent among older patients,' says Dr Richard Griffiths, a consultant anaesthetist at Peterborough City Hospital. 'It can affect those aged 30 to 40, too, though this is not common, but we don't really understand why it happens.'

Some think it may be a reaction to the surgery.

'Surgery is a form of trauma and evokes the body's protective response, which is inflammation,' says Professor Pandit. 'One theory is that inflammation promotes POCD.'

A small Scandinavian study of patients over 70 found the risk also decreased slightly for those who had day-case surgery rather than staying overnight in hospital.

Some experts, such as Dr Timo Pales - who has just retired from the department of anaesthesiology at the regional hospital in Biel, Switzerland - believes there is 'good science' to link general anaesthesia to problems beyond decline of brain power. He says it can reduce the activity of the immune system, too.

'The longer the surgery and the older you are, the higher the risk of developing the condition.'

One theory is that during general anaesthesia the nerves are 'awake' and feeling pain. The body produces stress hormones as a result and this impacts on the immune system.

'That is why if someone with cancer has a general anaesthetic it can spread because the general anaesthesia has a suppressant effect on the T-cells - these are some of the chief “killer” cells of the immune system,' says Dr Pales.

'We are aware of the fact there may be an association with general anaesthesia and the immune system,' says Simon Marsh, a consultant surgeon at The London Breast Clinic.

Indeed, a review of 14 studies published this year in PLOS One concluded 'epidural anaesthesia and/or pain relief might improve the survival chances of patients with cancer undergoing surgery (especially colorectal)'.

They suggested this may be due to the effect general anaesthetic has on the immune system.

Avoiding a general anaesthetic also improves pain relief, says Dr Sanders. This limits the amount of strong painkillers, such as morphine, required afterwards.

'This is important as anaesthetics, benzodiazepines (used for sedation) and opioids (such as morphine) all exert significant immune effects.'

Dr Pales says Britain is behind the rest of the world in recognising the benefits of regional or epidural anaesthesia. This involves giving local anaesthesia around a nerve, blocking pain signals to the brain.

Harmful: Some experts believe the use of general anaesthesia can reduce the activity of the immune system

'In Scandinavia and Switzerland, we have favoured regional anaesthesia for decades, but Britain is more conservative,' he says.

A report in the U.S. journal Anesthesiology this year reviewing 400,000 patients who had a hip or knee replacement found those who had regional rather than general anaesthesia had a lower death rate and had better health after the operation generally than those who had a general anaesthetic.

'For me this study is a real game changer,' says Dr Griffiths. A regional anaesthetic may cut the risk of post-operative cognitive decline, but is not suitable for all surgery - you can't anaesthetise the trunk in isolation for surgery on the heart or the abdomen, for example.

Nor is regional anaesthesia suitable for children - it is too distressing for them to deal with an operation while awake.

Yet a variety of studies have found a link between general anaesthesia in the young and problems with speech or learning difficulties.

Don't forget: Local anaesthesia, which is safer and more effective, is used far more frequently than general anaesthesia in most countries

One published in Paediatrics in 2011, which involved 5,000 children, concluded that: 'Repeated exposure (more than once) to general anaesthetic before the age of two was a significant risk factor for the later development of learning difficulties.'

The thought is that the anaesthesia may lead to the death of neurons or brain cells in the very young.

'No one has studied a human child's brain for this, but scientists have looked at those of young primates and rodents and they found cell death,' says Dr Randall Flick, an associate professor of anaesthesia and paediatrics at the Mayo Clinic Children's Centre in America.

Yet Dr Flick says this research has not changed the way he practises 'at all'.

'When parents ask: “Is general anaesthesia safe?” I say: “I can't yet judge if it is a real threat or not.” The risk is probably very small, but the truth is we don't know the magnitude of it.'

Dr Sanders says patients who have concerns should see their GP or speak to their anaesthetist about different techniques.

Tony Hewitt-Miles wishes he had done this.

'Rita could not have lived with a broken leg, but if someone had said “Your wife is now 80 and there is a possibility that general anaesthesia might affect her,” we might have taken another option, such as a regional anaesthetic.

'As it is, Rita has never been the same since and I am losing hope that she ever will be.'